1. Field of the Invention
The invention concerns a method and a control sequence determination device to determine a magnetic resonance system control sequence. Moreover, the invention concerns a method to operate a magnetic resonance system using such a magnetic resonance system control sequence; as well as a magnetic resonance system with a radio-frequency transmission device, with a gradient system and a control device that is designed in order to emit a radio-frequency pulse train to implement a desired measurement on the basis of a predetermined control sequence, and to emit a gradient pulse train in coordination therewith via the gradient system.
2. Description of the Prior Art
In a magnetic resonance tomography system (shortened to “magnetic resonance system”), the body to be examined is typically exposed to a relatively high basic magnetic field (known as the B0 field)—for example of 3 or 7 Tesla—produced by a basic field magnet system. A magnetic field gradient is superimposed on the basic magnetic field with a gradient system. Radio-frequency excitation signals (RF signals) are then emitted by a radio-frequency transmission system with suitable antenna devices, which leads to the nuclear spins of specific atoms or molecules of an examination subject being excited to resonance by this radio-frequency field so as to be flipped by a defined flip angle relative to the magnetic field lines of the basic magnetic field. This radio-frequency excitation, and the resulting flip angle distribution, is designated in the following as a nuclear magnetization, or “magnetization” for short. Upon relaxation of the nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated by the spine that are received by suitable reception antennas, and then are processed further. Finally, the desired image data can be reconstructed from the raw data acquired in such a manner. The emission of the radio-frequency signals (known as the B1 field) for nuclear spin magnetization most often takes place using an antenna known as a “whole-body coil” (or “full-body coil”) that is permanently arranged around the measurement space (patient tunnel) in the apparatus. Reception of the magnetic resonance signals most often takes place, however, using reception coils known as local coils that are positioned closer to the body of the patient. In principle, however, a reception of magnetic resonance signals can take place with the whole-body coil and/or transmission of the RF signals can take place with the local coils.
A control sequence with a radio-frequency pulse train that is to be emitted and a gradient pulse train that is to be switched in coordination therewith (with matching gradient pulses in the slice-selection direction, in the phase coding direction and in the readout direction, frequently in the z-direction, y-direction and z-direction), as well as additional control specifications, is defined for a defined measurement (data acquisition procedure) in a specification that is known as a measurement protocol. This measurement protocol can be created in advance and can be retrieved (for example from a memory) for a defined measurement, and may possibly be modified by the operator on site. During the measurement, the control of the magnetic resonance system then takes place wholly automatically on the basis of this control sequence, with the control device of the magnetic resonance system reading out and executing the commands from the measurement protocol.
In order, to generate the control sequence, it is typical for the individual RF pulse trains (i.e. the RF trajectories) for the individual transmission channels are determined over time in an optimization method, depending on a fixed “k-space trajectory” that is typically provided by a measurement protocol or individually by an operator. In order to implement such a “transmission k-space trajectory” (called only “k-space trajectory” or “trajectory” for short in the following), locations for entry of raw data in k-space that are selected in the desired order by adjusting the individual gradients at specific times. K-space is the partial frequency domain, and the trajectory in k-space describes the path in k-space that is traversed over time upon emission of an RF pulse by the aforementioned switching of the gradient pulses. By adjusting the k-space trajectory, it can be determined at which spatial frequencies specific amounts of the RF energy are applied (effective).
Additional, currently measured B1 maps that respectively indicate the spatial B1 field distribution for a specific antenna element, and a B0 map that represents the off-resonances or deviation of the B0 field from the actual desired, homogeneous B0 field (i.e. the actually sought Larmor frequency), with spatial resolution, can be taken into account in the optimization method to generate the control sequences. Moreover, for the planning of the RF pulse series the user often provides a target magnetization, for example a desired flip angle distribution. The RF pulse series that is necessary to achieve entered requirements is then calculated with a suitable RF pulse optimization program so that the target magnetization is achieved. In many cases, this is an optimally homogeneous magnetization in the desired field of view (FoV) that is to be examined, or the desired region to be excited (FoE—Field of Excitation).
In more recent methods it is possible to also selectively excite entire defined regions (for example two-dimensional regions) within a slice, meaning that a non-homogeneous target magnetization is deliberately sought.
One possibility to determine a two-dimensional radio-frequency pulse sequence (known as a “2DRF pulse”) in the previously described manner is described in the article, “Magnitude Least Square Optimization for Parallel Radio Frequency Excitation Design Demonstrated at 7 Tesla With Eight Channels” by K. Setsompop et al., Magn. Reson. Med. 59: 908 through 915, 2008. The transverse target magnetization is represented in a linear matrix equation system from the spatial coil profiles and the multichannel radio-frequency pulse series, into which information can also be entered about the present B0 maps and B1 maps and the k-space trajectory that is used. This equation system is then numerically solved for a specific, predetermined radio-frequency pulse series. Examples of such radio-frequency pulse series with selective excitation are echoplanar or spiral trajectories.
Such one-dimensional, two-dimensional or multidimensional k-space trajectories for a selective excitation have a higher complexity in relation to the typically used trajectories with constant gradients that are used for a simple, slice-selective excitation. Due to this higher complexity, a higher risk of artifact formation in the images also exists, for example because such pulses can already be significantly longer. Furthermore, an increased risk exists that hardware radio-frequency limitations and/or limitations with regard to the radio-frequency exposure of the patients (for example limitation of the SAR=Specific Absorption Rate or of the SED=Specific Energy Density) are exceeded. This is particularly the case for trajectories that are presently used most often, with equidistant curves of adjacent tracks (for example spiral trajectories) whose spiral revolutions proceed with a constant pitch, or rectilinear trajectories (for example EPI trajectories) with equally spaced, straight-line neighboring tracks running in parallel.
However, exceeding maximum allowable voltage peaks or radio-frequency power limits can lead to damage to the device hardware, or to the situation that radio-frequency pulses are limited later in the measurement on the basis of safety checks (in particular are reduced in power), which then in turn leads to unsatisfactory excitation results and consequently worse image data. For this reason, complicated k-space trajectories are calculated automatically within the scope of optimization methods, with maximum allowed powers, SAR and/or SED limit conditions, etc. are also being taken into account in the optimization methods. However, such calculations are very complex and time-consuming, and the results are sometimes unsatisfactory or unstable.